This proposed prospective randomized clinical trial will test the hypothesis that dietary fat intake reduction as an adjuvant breast cancer intervention will reduce disease recurrence and increase patient survival for post-menopausal women with localized disease. Potential mechanisms of action for this therapy include the influence of dietary fat on circulating steroid hormone concentrations, prostaglandin synthesis, immune function, regulatory gene expression, and cancer cell membrane structure and function. This study will enter a total of 2,000 post- menopausal women (>50 years of age) with a histologic diagnosis of localized breast cancer (stage I-IIIa) who have a dietary fat intake .25% of calories at baseline assessment. Following primary surgical therapy, all patients will receive systemic tamoxifen therapy with or without chemotherapy (CMF or AC). Chemotherapy will be prospectively defined for clinical subgroups utilizing a central randomization at the Statistical Coordinating Unit. Patients will be randomly assigned to either an intensive intervention group (IIG) or a non-intensive intervention group (NIG). The IIG will receive individualized programs targeted to reduce dietary fat intake to 15% of calories. The NIG will be instructed in the 1990 U.S. Dietary Guidelines (30% of calories from fat). Patients in both dietary groups will be counselled regarding the nutritional adequacy of their diets for RDA nutrients. Dietary fat intake will be estimated with serial unannounced food recalls. Serum fatty acid concentration profiles and weight change will also assess dietary adherence. All patients will be followed for disease-free survival as primary study endpoints and overall survival. This multidisciplinary study will involve 20 institutions in a multiinstitutional trial and will include an Administrative Unit, a Nutritional Coordinating Unit, a Statistical Coordinating Unit, three Regional Nutrition Centers to facilitate nutritional intervention, and participating Clinical Units. The successful completion of this study will provide a definitive evaluation of the effect of dietary fat reduction on breast cancer patient outcome.